Ablative method for forming RF ceramic block filters

ABSTRACT

A method of forming metallization patterns on a block of dielectric material wherein the entire surface area of the dielectric block is encased with a conductive material and unwanted conductive metal is ablatively etched from a designated surface area of the dielectric block to form desired metallized circuit patterns.

CROSS-REFERENCE TO RELATED APPLICATION

This application now U.S. Pat. No. 6,462,629 divisional of applicationSer. No. 09/333,71 filed Jun. 15, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to electrical filters and relatesparticularly to filter apparatus and a method of forming so-calledceramic filters and duplexers.

2. Description of Related Art Including Information Disclosed Under 37CFR 1.97 and 1.98

RF ceramic filters are well known in the art. They are constructed ofblocks of ceramic material that are typically coupled to otherelectronic circuitry through discrete wires, cables, and pins coupled toconductive connection points on external surfaces of the blocks. Theyare also used to construct duplexers and other electronic components.

Ceramic block filters are used in wireless communication products. Threemajor process steps in the manufacture of these filters are: (1) thecapacitive element pattern generation, (2) I/O pad generation, and (3)tuning the filter to the proper operating frequency. The function of thecapacitive element pattern on the filter is to approximate the RFresponse required by the customer. The function of the I/O padgeneration operation is to provide the interface from the filter to thewireless communication product. The function of the tuning operation isto finally adjust this approximated RF response to meet the exactcustomer requirements for the desired response.

In the prior art, a ceramic block is sintered and then silver metallicpaste is placed on all sides of the block EXCEPT for those sides thatrequire a defined electrical circuit such as a capacitive elementpattern or input/output (I/O) pads. On those sides, the silver metallicpaste is applied to the ceramic block in the form and shape of thedesired pattern and the I/O pads (through a well-known screen printprocess technology or abrasion technology). A heating process causes themetallic paste to solidify with the pattern(s) and/or I/O pads generallyin their proper location.

However, this screen print process does not have the dimensionalaccuracy desired in the plating of the capacitive element filter patternand other filter elements. The capacitive element filter patterndimensional accuracy on the ceramic block filters is required to be fourtimes (4×) as accurate at 1.8 GHz as at 900 MHz.

Thus, in order to complete the product, the capacitive element filterpattern must be further tuned to meet exact customer specifications.This can be accomplished by adding excess metallic paste adjacentcircuit pattern features and then using some method to sinter themetallic paste, forming an integral addition to the pattern or pads or,in some cases, material can be removed for tuning. By applying a signalto the input and monitoring the output signal during this process, theoperator can terminate the addition of material to the pads or terminalswhen the output signal indicates that proper tuning has been achieved.

Thus, in U.S. Pat. No. 5,198,788 fine tuning of ceramic filter metallicterminals or pads is disclosed. In this patent, the ceramic block iscoated on all sides but one (and perhaps a portion of an adjacent side)and on the uncoated side (and portion), silver metallic paste is formedin the general shape of the desired electronic terminals, pattern, orpads. The silver metallic paste is heat-treated to form a rigid coating.Additional silver metallic paste is then placed adjacent andelectrically contacting the formed terminals or pads and, while an inputsignal is being monitored on the output terminal, a laser beam is usedto scan the silver metallic paste to sinter it and form a solid additionto the terminal or pad until the proper electrical characteristics ofthe device is obtained. Thus, this patent relates to the addition ofmetal to the already generally formed terminals, pattern, or pads totune the circuit.

In U.S. Pat. No. 5,769,988 there is disclosed a method of manufacturinga ceramic electronic component having a dielectric ceramic and aconductor thereon containing silver as the main component. Byheat-treating the device, such as a dielectric resonator formed withthat process, the “Q” value of the resonator is increased. In theprocess of heat-treating the device, it is disclosed that the device issubjected to a heat treatment at 400° C. or more in an atmospherecontaining 10% or less by volume of oxygen. Further, Kagata, et al('988) disclose “forming the conductive paste . . . in a pattern ofelectrodes” on the sintered dielectric ceramic substrate.

U.S. Pat. No. 5,162,760 also relates to electrical filters formed ofceramic blocks using abrasive or milling methods to remove metallizationor use various screen-printing techniques to apply conductive materialsonto the various surfaces of the ceramic blocks. In the '760 patent, alayer of conductive material is deposited on the surface of the blockand after the layer is successfully cured, portions of the conductivelayer are removed by any suitable milling machine such that the desiredconductive pattern is left on the surface. Both the conductive materialcoating the block and the dielectric material are removed from the blockin the areas that are milled. This device is limited in accuracy orprecision by the electrode dimension that can be formed with a millingmachine.

In U.S. Pat. No. 5,379,011 there is described a ceramic band-pass filterwith improved input/output isolation and having conductive materialremoved from the metallization of the block and the I/O pads aredeposited in those areas where the conductive metal had been removed.Again, in this patent, all six sides of the ceramic block are metallizedwith the exception of the top or upper surface and a portion of the sidesurface. Slots are formed, between the deposited input/output pads andadjacent metal in the ceramic material and thus, when not plated, variesthe dielectric between the input/output pads.

It would be desirable to have such a filter with good isolation betweenconductive areas and having greater dimensional accuracy of the filterpattern and I/O pads than the present art can provide.

SUMMARY OF THE INVENTION

In the present process, the ceramic block is formed in the usual manner.It has at least one planar surface. Then, instead of coating only thosesides where the pattern or I/O pads are not to be formed, the ENTIREceramic block is coated with a conductive metallic material. One exampleof such metallic coating is a paste well known in the prior art andcontaining an electrically conductive metal (such as silver, for exampleonly) and is then subjected to the necessary heat treatment to solidifythe metal. Other examples of conductive coatings include plating theceramic blocks with a conductive metal and the like.

An ablative method, such as the use of a scanning laser beam, is used toremove unwanted metallic material from at least one planar surface toform the desired capacitive element filter patterns. This differs fromthe abrasive method of the prior art or the screen printing of the priorart. The laser beam ablates both the metallization and a portion of theceramic block to form trenches that surround metallic filter componentsand create the pattern in the desired shape. The depth and width of thetrenches determine coupling capacitance of the filter and thus determineits operating frequency. The precision and repeatability of forming thetrenches with the lasing process allows greater accuracy andrepeatability of the capacitive element filter pattern and the otherfilter components. The more precise patterns allow for higher tunerates, higher factory yields, and more design margin for the productdesigners.

However, a well-known problem occurs as a result of the ablationprocess. During the lasing process, the ceramic material is adverselyaffected and the “Q” of the ceramic material is reduced to a point wherethe filter has no commercial value. Therefore a post-lasing,high-temperature heating process is required to restore the ceramic “Q”back to its approximate original value.

Since, during the ablation process, the capacitive element patterns andother filter components are being formed, a signal cannot be connectedto the input pad for monitoring at the output pad to see if thecapacitive element patterns and the other filter components being formedare of the correct dimensions. After the filters have been formed, suchsignals cannot be applied and measured to the product specificationbecause the ceramic block has such a reduced “Q” that they are only ageneralized representation of the signals that would be found in afinished product. Therefore, for a given product specification, atrial-and-error ablation process is used by continuing to makemetallized blocks with different dimension conductive patterns untilsignals representing the proper RF response range have been establishedto form a “reference” ceramic block. Since the lasing process isextremely precise and repeatable, great numbers of the reference devicecan then be produced and then a high temperature heating process is usedto provide the proper RF response once the appropriate patterns havebeen generated.

Thus, it is an object of the present invention to provide a method offorming electrically conductive metallization patterns on a ceramicblock that are electrically isolated from each other by a pattern ofdielectric material.

It is still further an object of the present invention to provide aceramic block that has its entire surface coated with a conductivematerial and to use a scanning laser beam to ablatively etch unwantedmetallic material and corresponding ceramic material from the ceramicblock and create trenches that form at least a portion of the pattern ofdielectric material that establishes the desired metallization pattern.

It is also an object of the present invention to ablatively etchunwanted metallic material in such a way that a portion of the ceramicblock is also removed sufficient to form trenches that electricallyisolate adjacent metallic areas formed by the ablative etching.

It is another object of the present invention to ablatively etchunwanted metallic material from a designated surface area of the ceramicblock to form trenches of dielectric material that create a desiredmetallization pattern, including inputs and output terminals.

It is still another object of the present invention to apply testsignals to the input terminal of a post-lased, pre-heated ceramic filterand to monitor the output signal to determine by trial and error when afilter having the desired electrical characteristics is obtained.

It is yet another object of the present invention to heat theablatively-etched ceramic block in an ambient atmosphere to restore the“Q” of the ceramic material.

Thus, the present invention relates to a method of forming ceramic blockmetallization patterns comprising the steps of encasing the entiresurface of a ceramic block, including at least one planar surface, witha conductive metal, such as a metallic paste, for example only,solidifying the conductive metallic paste into a metallic material, andablatively etching unwanted metallic material from a designated surfacearea of the ceramic block to form a desired metallization pattern,including input and output terminals if desired.

The invention also relates to a method of forming RF ceramic blockfilters comprising the steps of encasing the entire external surfacearea of a ceramic block with a conductive metallic material, using apattern of dielectric material to electrically isolate and formelectrically conductive circuit elements on the encased ceramic block,ablatively etching unwanted metallic material and a portion of theceramic block from a designated surface area of the encased ceramicblock to form at least one trench that forms at least a portion of thepattern of dielectric material and heating the ablatively-etched ceramicblock to increase the “Q” thereof.

The invention also relates to trench filters and duplexers in which eachconductive element formed in a ceramic block whose entire surface isencased in a conductive material, each of the conductive elements beingat least partially surrounded by a trench or recessed area extendingthrough the conductive material and into the ceramic block, the recessedarea having a predetermined depth and width to affect the couplingcapacitance of the filter or duplexer and thus control an operatingcharacteristic of said filter or duplexer.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the present invention will be more fullydisclosed when taken in conjunction with the following DetailedDescription of the Preferred Embodiment(s) in which like numeralsrepresent like elements and in which:

FIG. 1 is a perspective view of a prior art ceramic band-pass filteradapted for tuning by adding material thereto;

FIG. 2 is a perspective view of a ceramic block of the present inventionentirely encased with an electrically-conductive material such as ametallic paste;

FIG. 3 is a top view of a duplexer formed by the method of the presentinvention;

FIG. 4 is a side view of the duplexer of FIG. 3 illustrating theconducting pads or terminals formed thereon and the trenches formedaround the conductive filter elements;

FIG. 5 is a flow diagram illustrating the novel steps of the presentinvention;

FIG. 6 is a graph illustrating the frequency response of a plurality ofprior art filters shown in FIG. 1;

FIG. 7 is a graph illustrating the frequency response of a plurality offilters formed by the present inventive process after lasing but beforehigh temperature heat processing; and

FIG. 8 is a graph illustrating the frequency response of a plurality offilters formed by the present inventive process after firing to restorethe “Q” of the ceramic.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 1 is a perspective view of a prior art ceramic band-pass filter.Filter 10 is formed of a ceramic block with some surfaces having aconductive metal plate thereon and includes a top face 12, a bottom face14, side faces 16 and 18, and end taces 20 and 22. Filter 10 furthercomprises parallel cylindrical bores 24 and 26 that openly extendbetween top face 12 and bottom face 14. Regions of the ceramic blocksurfaces, such as top surface or face 12, are screen-printed, in awell-known manner, with conductive metal material such as a silver pasteto form metallic elements of the filter leaving the bare ceramic surfacematerial of the ceramic block between the filter elements. The printedelements 36 and 38 rise above the top face of planar surface 12 of theceramic block and include an input pad 28 and an output pad 30 that maywrap around between top face 12 and side face 16. The wraparoundconfiguration is particularly adapted for surface-mount connections whena filter is subsequently incorporated into an electronic package. Theside faces 16 and 18, bottom face 14, and end faces 20 and 22 arecovered with a continuous metal plate that forms a ground element 32. Atface 16, ground plate 32 is separated from input pad 28 and output pad30 by bare ceramic regions 34 and 35 to prevent electricalshort-circuiting. These bare ceramic regions 34 and 35 are created whenthe I/O pads are screen-printed on the ceramic substrates. Through-holes24 and 26 are coated with conductive metal that extends onto the topface 12 to include the resonator pads 36 and 38 that are surrounded bybare ceramic surface. In accordance with the prior art, pads 36 and 38include notches 40 and 42 for tuning the filter.

The prior art filter as shown in FIG. 1 has a thick film of metallicpaste applied to the major surfaces, except for faces 12 and 16 and alsoto through-holes or cavities 24 and 26 by spraying, screen-printing, orother well-known process. The film of conductive paste is screen-printedin the desired pattern onto top surface 12 and side surface 16. Thefilter can be tuned as described more fully in U.S. Pat. No. 5,198,788.

Thus, in the prior art, a ceramic block is sintered and then silvermetallic paste is placed on all sides of the block except for thosesides that require a defined capacitive element pattern or input/output(I/O) pads. On those sides, the silver metallic paste is applied to theceramic block in the form and shape of the desired pattern and pads(through a well-known screen-print process technology) thus formingconductive surfaces that are above the planar surface 12 of the ceramicblock. A heating process causes the metallic paste to solidify with thepatterns and/or I/O pads generally in their proper locations. However,this screen-print process does not have the dimensional accuracy desiredin the plating of the capacitive element filter pattern. The capacitiveelement filter pattern dimensional accuracy on the ceramic block filtersis required to be four times greater (4×) at 1.8 GHz as required at 900MHz. The dimensional accuracy of the screen-printing process is capableof producing acceptable 900 MHz filters. However, above 900 MHz, thefrequency response continually degrades thereby producing smaller andsmaller numbers or quantities of filters having an acceptable frequencyresponse. Therefore, in order to complete the product, the capacitiveelement filter pattern must be further tuned to meet exact customerspecifications. This can be accomplished by adding excess metallic pasteto adjacent pattern features (e.g. slots 40 and 42 in FIG. 1) and thenusing some method to sinter the metallic paste, thus forming an integraladdition of metal to the pattern or pads. By applying a signal to theinput terminal and monitoring the output signal at the output terminal,the operator can cease the addition of material to the pads or terminalswhen the output signal indicates that proper tuning has been achieved.See U.S. Pat. No. 5,198,788 incorporated herein by reference.

FIG. 2 illustrates a perspective view of a ceramic block of the presentinvention entirely encased with a conductive metal. The conductive metalmay be plated thereon or may be formed with an electrically-conductivemetallic paste that has been heated to solidify it. The block 40 has aninternal ceramic portion 39 entirely encased, on all of its outsidesurfaces, including capacitive adjusting through-holes 43 and 45, withthe conductive metal. Only two through-holes are shown but, obviously,more can be included as shown in FIG. 3. Thus, instead of coating allsides except for the sides where the pattern or I/O pads are to beformed, the entire ceramic block is encased with a conductive metal suchas, for example only, an electrically-conductive metallic paste (wellknown in the art) that is subjected to the heat treatment to solidifythe metal.

A duplexer 42 using filters formed with the process of the presentinvention is illustrated in FIG. 3. It comprises a transmitter portion44 and a receiver portion 46. It includes an I/O pad 48 for thetransmitter (not shown), an antenna pad 50 for coupling signals to andfrom the transmitter portion 44 and the receiver portion 46, and an I/Opad 52 for connecting signals to the receiver (not shown). Thetransmitter portion 44 of the duplexer 42 includes resonators andassociated circuit elements 53, 54, 55, 56, and 58 while the receiverportion 46 uses resonators and associated elements 60, 62, 64, 66, and68. FIG. 3, before the circuit elements are formed thereon, looks likethe block in FIG. 2 and the upper surface 41 has metal and correspondingceramic ablatively removed therefrom to provide a pattern of dielectricmaterial that forms, and electrically isolates, the circuit elements 53,54, 55, 56, 58, 60, 62, 64, 66, and 68. It should be noted that the term“electrically isolates” as used herein means “no direct electricalconnection”. That is, there no longer is electrical continuity orconnection between “isolated” conductive elements. There may be,however, electrical “coupling” between adjacent elements by means ofelectromechanical coupling (a piezoelectric effect) or alternatingcurrent (AC) coupling as through a capacitive effect. Note that in areas70, 72, 74, and 78 that the metal and a corresponding portion of theceramic material have been removed by an ablative process. This ablativeprocess, preferably performed with a scanning laser beam, removes notonly the conductive metal but also a corresponding portion of thedielectric block to form “trenches” or recessed areas 70, 72, 74, 76,and 78. These trenches have a depth and a width in any given portion ofthe filter pattern that affects the coupling capacitance betweenadjacent metallic surfaces in a well-known manner and thereby affectselectrical characteristics of the filters such as frequency of operationand impedance. Of course, if desired, some of the conductive elementscould be formed by the prior art method of screen-printing and the outeredges thereof trimmed by a laser beam according to the presentinvention, to accurately control the conductive elements. The remainderof the patterns of dielectric material could be formed by the presentnovel process. In such cases, however, the trench forms at least 10%,and preferably in the range of about 70% to about 90%, of the pattern ofdielectric material electrically isolating the electrical circuitpattern elements.

Thus, for the duplexer 42 shown in FIG. 3, it is formed from a block ofdielectric material having multiple surfaces including at least oneplanar surface such as the block illustrated in FIG. 2. It also has alayer of metallic material coating all sides of the multiple surfaces ofthe block of dielectric material also as shown in FIG. 2. A ground plane80 is formed by at least some of the metallic surfaces as illustrated inFIG. 3 by conductive material 82. A receiver filter 46 is formed in afirst area of at least one planar surface of the block of dielectric andincludes a first plurality of conductive elements 60, 62, 64, 66, and68. A trench or recessed area 74, 76, and 78 surrounds each of theconductive elements to electrically isolate them and is recessed to apredetermined depth and has a predetermined width to cause a capacitivecoupling between the conductive elements and the ground plane thatdetermines an operating characteristic of the receiver filter such asthe operating frequency.

In like manner, the transmitter filter 44 is formed on a seconddifferent area of the at least one planar surface of the dielectricblock shown in FIG. 2 and includes a second plurality of conductiveelements 53, 54, 55, 56, and 58 that has the trenches 70, 72, and 74that surround each of the plurality of conductive elements forming thetransmitter portion 44. Again, these trenches are ablatively removed andextend through the conductive metallic material and into the dielectricmaterial. Thus, a second capacitive coupling is formed between theconductive elements of the transmitter 44 and the ground plane that,again, provides a capacitive coupling that is determined by the depthand width of the trenches or recessed areas and determine the operatingfrequency of the transmitter filter. First and second terminals 48 and52 for the transmitter and the receiver, respectively, are formed on thetop and side of the coated dielectric block such as shown as in FIG. 1and in FIGS. 3 and 4. A third terminal 50 receives an antenna connectionand is electrically coupled to the receiver filter and the transmitterfilter in a well-known manner. It conveys the RF signals between theantenna and the RF receiver and the RF transmitter.

One or more through-holes such as 43 and 45, well known in the priorart, are plated with conductive matter to create resonate circuitelements.

If it is desired to fine-tune the filter, metal can be ablativelyremoved from areas such as, for example only, 88, 90, 92, and 94 tofine-tune the transmitter. The receiver 46 can be tuned in like manner.In this case, again, the ablative etching will remove both the metalplate and the ceramic to a desired depth.

FIG. 4 is a side view of the duplexer of FIG. 3. All of the elementsshown therein are enlarged and not to any proportion. However, it can beseen in FIG. 4 that on the side 82 the transmitter terminal 48 extendsdown the side of the block and an area of conductive metal 82, shown at84, is removed from around the terminal 48 thus exposing the ceramic 86.In like manner, the conductive elements 53, 54, 55, 57, and 58 shown ontop of the ceramic have been formed by the creation of the trenches orrecesses as set forth in the discussion of FIG. 3.

The same construction can be shown in relation to the third terminal 50and the second terminal 52.

The important matter to consider here is that the block has beenentirely coated with a conductive metal and that the patterns have beenformed on the surface by ablatively etching away unwanted material to adesired depth and width to provide the proper operating characteristicsof the filter. It will be noted in FIG. 4 that the surfaces of theconductive elements are at the same elevation as the surface 41 of FIG.2 and FIG. 4. In other words, there is no formation of conductiveelements on top of the ceramic but rather a deletion of metal as well asof the ceramic block by using the ablative method. This is entirelydifferent from the prior art filter elements formed on top of and abovethe surface of the dielectric block and the terminals on the side of theblock are formed by screen printing.

As stated earlier, however, a portion of the conductive metallicmaterial may be formed, as in the prior art, with screen printing andthen the remainder of the conductive metallic circuit patterns may beformed by the ablative etching as heretofore described.

FIG. 5 discloses a flow chart illustrating the novel steps of thepresent invention. At step (A), the entire surface of the ceramic blockis coated with a conductive metal, preferably a metallic paste. At step(B), the conductive metal is solidified into a metallic material such asby heating the metallic paste and to cause it to adhere to thecorresponding ceramic block. At step (C), unwanted metallic material anda portion of the corresponding ceramic block is ablatively etched awayto form the desired metallic pattern on the block including I/Oterminals. Of course, I/O terminals could be added on the side by someother method, such as screen printing as explained earlier and couldthen be electrically coupled to the metallic patterns that are formed onthe top surface by ablatively etching.

At steps (D) and (E), the prototype filter is electrically checked. Atstep (D), an input signal is connected to the input terminal and, atstep (E), an output signal at the output terminal is monitored todetermine the electrical characteristic of the ablated ceramic block.

At step (F), steps (A) through (E) are repeated until a ceramic blockhaving approximate desired electrical characteristics is obtained. Atthat time, the ceramic block filters can be mass-manufactured and, atstep (G), heated in an ambient atmosphere to increase the “Q” thereof.Of course, an ablated ceramic block could be heated to restore the “Q”and then, if the proper filter response is found, the ceramic blockfilters could be mass-manufactured, heated, and quality control providedto ensure filters having the desired characteristics.

FIG. 6 is a graph of the frequency response of a plurality of filtersformed with the prior art process of printing the circuit on the ceramicblock surface. Note that the peak insertion loss is −1.2 dB and thefrequency standard deviation is 2.63 MHz. It can be seen that on eachend of the curve that there is a great deal of variation and responseamong the filters, thus changing the bandwidth of the devices.

FIG. 7 is a graph of a plurality of filters made with the novel methodof the present invention. Note how closely they all respond all acrossthe graph and especially across the bandwidth at the top. This graphillustrates the frequency response of a plurality of the devices afterthe lasing process to create the filter circuit elements but before theyhave been heated to restore the “Q”. Therefore, notice that the peakinsertion loss is 5.4 dB. However the center frequency deviation hasbeen reduced to 0.85 MHz.

FIG. 8 is a graph of the same ceramic filters made with the process ofthe present invention and shown in FIG. 7 after they have been heated torestore the “Q”. Again, notice how closely they replicate each other andthe peak insertion loss has now been reduced to −1.2 dB and centerfrequency standard deviation is 0.75 MHz.

Thus, it can be seen there is a significant improvement in the operationof the filters made under the process of the present invention.

Thus, there has been disclosed a novel process and apparatus in whichprocess two major steps are included. The first is that all of thesurfaces of the ceramic block are covered with plated metal and, second,that at least some of the desired filter circuit patterns are formed byablatively etching unwanted materials from one or a portion of a secondsurface of the plated ceramic block ablating both the metal andunderlying ceramic.

The first novel step differs from the prior art in that in the prior artone or more surfaces are left uncoated and then the patterns are platedthereon, thus the patterns have a top surface that rises above the topsurface of the ceramic block. In the present application, because themetal surfaces of the patterns formed are coplanar with the other metalsurface that was not removed on the block, tuning simply requiresremoving additional metal from the metal circuit patterns. Further, theablative technique used to remove the metal also removes a portion ofthe dielectric, thus creating a trench or recessed area around each ofthe conductive elements of the filter circuit pattern to electricallyisolate them. The width and depth of these recessed areas determine theoperating parameters of the filter such as the operating frequency.

The second step of ablatively etching away unwanted material anddielectric not only to form the metal filter circuit patterns but alsoto determine the frequency response of the filter is novel since, in theprior art, a laser was used only to add metal to tune the filtercircuits.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed.

What is claimed is:
 1. A method of manufacturing an RF ceramic filtercomprising the steps of: forming a block of ceramic material having anouter surface with least one pair of opposing sides and defining aplurality of through holes extending between the opposing sides;covering the block with a conductive coating; heat treating the coatedblock; and ablatively etching a selected area of the heat-treated coatedbloc to form a pattern of metallized and unmetallized areas on theblock, wherein the step of ablatively etching is carried out such thatthe unmetallized areas are recessed into the block of ceramic material.2. The method according to claim 1 further comprising the step of heattreating the patterned block.
 3. The method according to claim 1 furthercomprising the step of heat treating the patterned block to atemperature sufficient to reduce the filter insertion loss.
 4. Themethod according to claim 1 wherein the step of covery ring the blockwith a conductive coating includes contacting the block with a silverpaste.
 5. The method according to claim 1 wherein the step of ablativelyetching the block is carried out using a laser beam.
 6. The methodaccording to claim 1 wherein the step of ablatively etching the block iscarried out using a scanning laser.
 7. A method of manufacturing an RFceramic filter comprising the steps of: providing a ceramic block havingan outer surface with at least one pair of opposing sides and defining aplurality of through holes extending between the opposing sides;encasing the block with a conductive coating; heat treating the coatedblock; ablatively etching the conductive coating and a portion of theceramic block from selected areas of the heat-treated coated block toform a pattern of metallized and unmetallized recessed areas on theblock; and heat treating the patterned block.
 8. The method according toclaim 7 wherein the step of ablatively etching the block is carried outusing a scanning laser.
 9. The method according to claim 7 furthercomprising the step of heat treating the patterned block to atemperature sufficient to reduce the filter insertion loss.
 10. Themethod according to claim 7 wherein the step of ablatively etching theblock is carried out using a laser beam.
 11. A method of manufacturingan RF ceramic filter comprising the steps of: providing a block ofceramic material; encasing the block with a conductive coating; heattreating the coated block; ablatively etching with a laser selectedareas of the heat-treated coated block to form a pattern of unmetallizedrecessed areas and unablated metallized areas on the block; and heattreating the patterned block.
 12. A method of manufacturing an RFceramic filter comprising the steps of: (a) providing a ceramic blockhaving an outer surface with at least one pair of opposing sides anddefining a plurality of through holes extending between the opposingsides; (b) encasing the black with a conductive coating; (c) heattreating the coated block; (d) ablatively etching with a laser theconductive metal coating and a portion of the ceramic block fromselected areas of the heat-treated coated block to form a pattern ofmetallized and unmetallized recessed areas on the block, wherein thepattern of metallized and unmetallized recessed area includes atransmitter pad, an antenna pad and a receiver pad; repeating steps (a)through (d) to make a plurality of patterned blocks and thereafter heattreating the plurality of patterned blocks.